1. Technical Field
This invention relates to pigment compositions and in particular pigment compositions including titanium dioxide.
2. Art Background
Titanium dioxide is a pigment that is extensively utilized in a variety of applications. Generally the pigment is combined with a binder such as a latex, acrylic, vinyl acetate, alkyd, polyester, polystyrene, poly(methylmethacrylate) (PMMA), polyvinylchloride (PVC), melamine, cellulose and/or cellulose derivative in an emulsion or dispersion that is employed to produce a final product e.g., a painted board, colored plastic, or paper. For example, titanium dioxide is utilized as the pigment in white paint, in interior and exterior house paint, automotive paints and coatings. Titanium dioxide was first commercially produced by extracting titanium ore, such as Ilmenite (FeTiO.sub.3), with concentrated sulfuric acid to form a liquid sulfate solution, concentrating this solution, by evaporating water, separating the FeSO.sub.4 precipitate, hydrolyzing the solution to precipitate hydrated titanium dioxide, and baking at 800.degree. to 1000.degree. C. to form a cake. The resulting cake of agglomerated material is then mechanically treated to disaggregate agglomerates to smaller particles that are suitable for use. Although the titanium dioxide obtained by this procedure was employed for a substantial time, it had significant associated problems. These problems were generally correlated with relative broad particle size distribution and with the presence of impurities, particularly iron, that remained in the titanium dioxide during production. Generally, these impurities were homogeneously distributed in the titanium dioxide particles. Strong efforts were made to reduce the iron content and commercial limits of no more than 200 ppm by weight were established. (See H. Rechmann, Berichte Der Bunsengesellschaft fur Physikalische Chemie, 71, 277-285 (1967).) The impurities cause, because of their strong light absorption in part of the visible spectrum, a significant decrease in the reflectivity of the particles and, in turn, a substantial deviation from "white" color. Thus, great effort was focused on eliminating these impurities. The broad particle size distribution also reduces the pigments efficacy to scatter visible light. Particles of 0.2-0.3 .mu.m diameter are most efficient for scattering in the visible.
To alleviate these problems, a procedure was developed to produce a more uniform, purer titanium dioxide. This method described in Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd edition, Vol. 23, pages 143-149, in U.S. Pat. No. 2,833,627, 4,462,979, and in other patents mentioned in this latter patent (which are all hereby incorporated by reference), involves reaction of titanium tetrachloride with oxygen in a 1500.degree. C. flame to produce a particulate soot. The resulting titanium dioxide is not only substantially purer, but also is formed in uniform, appropriately sized particles, i.e., particles in the range 0.18 .mu.m to 0.26 .mu.m. Since appropriately sized particles are directly obtained, mechanical processing and its concomitant costs are eliminated.
Although this improved procedure for producing titanium dioxide yields substantially improved purity, binder degradation due to exposure to sunlight still occurs. To improve performance, the titanium dioxide is coated with a passivating, dielectric layer, e.g., silicon dioxide or aluminum oxide. (Generally a material with a resistance to electron or hole transport above 10.sup.4 ohms is desired.) The coating, it is believed, prevents photogenerated electrons and holes in the titanium dioxide from directly or indirectly reacting with the binder. Although stability is enhanced through this coating procedure, the enhanced stability is purchased at the expense of 1) the increased cost associated with the coating process, and 2) decreased reflectivity associated with the coating material. Nevertheless, the stability of such material is quite acceptable and it is presently being extensively utilized in commercial products.